In failing hearts with discoordinate contraction, the late-activated territory, which is under highest regional wall stress (lateral endocardium), exhibits reduced expression of key excitability and calcium handling proteins and gap junction channel proteins. Even without heart failure (HF), sustained discoordination induced by left-bundle branch block alters regional tissue electrophysiology. Our preliminary data demonstrates marked disparities between the anterior/septal (low load) and lateral (high load) walls with respect to conduction velocity and refractoriness, which are particularly pronounced in the subendocardium. We hypothesize that heterogeneous mechanical load in dyssynchronously contracting hearts exaggerates regional dispersion of conduction and refractoriness beyond that produced by HF alone. Further, significant regional and transmural differences in the functional expression of ionic currents exist between early and late-activated regions of dyssynchronously contracting ventricles. These differences are potently exacerbated by concurrent HF resulting in enhanced arrhythmia susceptibility, and are reversed by restoration of contractile synchrony. In order to test these hypotheses we will: 1. Characterize the regional electrophysiological substrate and susceptibility to ventricular arrhythmias in synchronously and dyssynchronously contracting failing and non-failing hearts in perfused myocardial wedges using electrical and optical mapping; 2. Characterize the effect of acute stretch and altered autonomic tone on the electrophysiological substrate and susceptibility to ventricular arrhythmias in dyssynchronously contracting failing and non-failing hearts versus synchronously contraction failing and control hearts; 3. Characterize regional cellular electrophysiology accompanying dyssynchrony in failing hearts. We will contrast APs and ionic currents (Na, Ca, K, Na-Ca exchanger) in myocytes isolated from inner versus outer layers of the lateral and antero-septal walls in normal and failing hearts with or without dyssynchrony, and relate changes to corresponding mRNA and protein expression, expression profiling and analysis of the sarcolemma subproteome as well as optical APs and CaT described in Aim #1.